Monks,

System Specs: Perl 5.10, Ubuntu 8.04, AMD 64x2, 4Gb RAM.

I am trying to understand why references that have gone out of scope are still hanging around.

Given the following code: 
package Top;

sub new {
  my $class = shift;
  my $s = bless { }, $class;
  $s->{bottom} = Bottom->new( $s );
  return $s;
}

#=====================================

package Bottom;

sub new {
  my ($class, $top) = @_;
  return bless { top => $top }, $class;
}

#=====================================

package main;

use strict;
use warnings 'all';
use Devel::Cycle;

my $top = Top->new();
find_cycle( $top );

[download]

I get the following output:
Cycle (1):
                   $Top::A->{'bottom'} => \%Bottom::B
                   $Bottom::B->{'top'} => \%Top::A

[download]

If I change the code for package main to this:
local $| = 1;
for( 1...1_000_000 )
{
  print "\r$_/1000000";
  my $top = Top->new();
}

[download]
I burn through 200Mb of RAM in 42 seconds.

Using Scalar::Util's weaken function takes care of the RAM issue, but the weakened references disappear too soon (i.e. before I get a chance to use them!).

Example:
package Top;
use Scalar::Util 'weaken';

sub new {
  my $class = shift;
  my $s = bless { }, $class;
  weaken($s);
  $s->{bottom} = Bottom->new( $s );
  return $s;
}

#=====================================

package Bottom;
use Scalar::Util 'weaken';

sub new {
  my ($class, $top) = @_;
  my $s = bless { top => $top }, $class;
  weaken($s);
  return $s;
}

#=====================================

package main;

use strict;
use warnings 'all';

my $top = Top->new();
my $bottom = $top->{bottom} or die "NO BOTTOM!";

[download]
Produces the output NO BOTTOM!

Where can I look for a straightforward description of Perl, circular references, weaken, etc?

UPDATE: - Now it burns through 52Mb RAM in 26 seconds. Not sure if that's good or bad. I would expect Perl to recycle the RAM. Is that expectation wrong? Sure, 52Mb is better than the original number of 200Mb, but I would like to see no leakage at all.

Is it possible to do this without leaking memory?

In reply to Memory leaks and circular references by jdrago_999

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